The invention relates to an arrangement for angle measurement by means of an angle sensor supplying two sensor signals which are phase-shifted by 90 degrees relative to each other and whose amplitudes are dependent on the temperature. In such arrangements, there is generally the problem that the sensor signals supplied by the angle sensor are very much dependent on temperature as regards their amplitudes.
An arrangement comprising an azimuth sensor is known from U.S. Pat. No. 4,739,560, in which the amplitudes of the sensor signals are corrected in dependence upon the temperature. This means that the temperature is determined and the change of the amplitudes of the sensor signals, expected as a result of this temperature, is compensated.
It is an object of the invention to improve the arrangement of the type described in the opening paragraph to such an extent that the faultless operation of the arrangement can be monitored.
This object is achieved in that the arrangement comprises a temperature sensor and a microprocessor, and in that the microprocessor computes amplitude values of the sensor signals expected in dependence upon a temperature value supplied by the temperature sensor, compares these values with the actual amplitude values of the sensor signals, and generates an error signal when the deviation between the expected and actual amplitude values exceeds a predetermined limit value.
The microprocessor which evaluates the sensor signals is associated with the temperature sensor. The temperature sensor supplies a temperature signal in dependence upon the temperature. Again in dependence upon this temperature signal, the microprocessor is capable of determining which amplitudes the sensor signals should have at the given temperature, because the dependence of the amplitude fluctuations of the sensor signals on the temperature is known.
The microprocessor can thus determine in advance which amplitudes the sensor signals supplied by the angle sensor should have at the given temperature. These expected amplitudes are compared with the actually determined amplitudes of the sensor signals. When the difference between these signals precedes a predetermined limit value, this indicates that there is an erroneous situation. In this case, the arrangement generates an error signal which may be used, for example, in subsequent circuit arrangements for suppressing the evaluation of the sensor signals. Thus the faultless operation of the arrangement including the angle sensor can be monitored.
The comparison between the actual and the expected amplitude values of the sensor signals can be most easily realized by comparing the maximum amplitudes of the two signals, as in an embodiment of the invention as defined in claim 2.
In accordance with a further embodiment of the invention as defined in claim 3, the microprocessor is formed in such a way that it uses the CORDIC algorithm for computing an angle, so that the radius determination provided anyway within the CORDIC algorithm can also be simultaneously used for determining the maximum values of the actual and current amplitude values. The CORDIC algorithm as such is known from, for example, xe2x80x9cDigitale Signalverarbeitung in der Nachrichtenxc3xcbertragungxe2x80x9d by Gerdsen and Krxc3x6ger, pp. 108 to 115. The computation of the expected amplitude values in dependence upon the temperature is realized by means of an analytical function.
The signals supplied by the angle sensor may be converted so that, as in a further embodiment of the invention as defined in claim 4, they can be converted, for example, from differential signals to asymmetrical signals. They can also be converted from analog signals to digital signals. Further conversions are also feasible; in any case, amplitude values of the sensor signals, dependent on the temperature, are obtained for the input signals of the microprocessor.
As defined in claim 5, the kind of temperature dependence may be stored in an EEPROM and used by the microprocessor for determining the expected amplitude values. When the temperature sensor supplies, for example, an analog signal, this signal is converted by means of an A/D converter into the digital range and applied to the microprocessor in order that this microprocessor can evaluate the signal. When in such a constellation, as in a further embodiment of the invention as defined in claim 7, also a multiplexer is provided, then it is possible to reconvert the analog output signal of the circuit arrangement time sequentially by means of the A/D converter into a digital signal and apply it to the microprocessor. This provides the possibility of controlling the analog output signal of the circuit arrangement. Due to this feedback, the microprocessor can steadily check whether the output signal actually corresponds to the values supplied. This allows an additional error control.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.